In surface analysis by techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) or secondary ion mass spectrometry (SIMS) for example, ion beams produced by an ion gun, particularly argon ion beams, are widely used to bombard the surface to be analysed. Such beams are used, for example, to clean surfaces of contaminants prior to analysis by the aforementioned techniques and/or to etch through surface layers to reveal the underlying structure to enable a depth profiled analysis of the surface to be made. It is difficult to produce argon ion beams with sufficiently low energy to avoid damage to some types of surface during the etching process, especially surfaces of organic materials, and consequent loss of chemical and molecular information about the surface. To avoid the aforementioned damage it is known that it requires the use of ion beams having particles with very low energy, typically less than 10 eV. This is not possible using a simple argon ion beam where such low energy beams are difficult to create and to focus due to space charge effects. Herein the term argon ion beam refers to a beam of ionised single argon atoms, (Ar+).
Improvements have been made by using C60, or other fullerene, ion beams, initially for SIMS and more recently for XPS analysis. Apparatus and methods using C60 ion beams are disclosed in GB 2386747 A and US 2008/0042057 A1. Other ion beams employing large molecules, such as coronene ion beams as well as ion beams using other polycyclic aromatic hydrocarbon (PAH) molecules, have also been employed as described in GB2460855 A. Such large molecules are generated with a high energy of typically 10 keV or more, but on impact fragment into smaller clusters and atoms which individually have very low energy. However, for at least XPS analysis, the use of a carbon based ion beam leads to surface deposition of carbon which can both halt the etching process and modify the chemistry of the sample thereby leading to erroneous results. Ion beams employing metal clusters have also been tried but large clusters are difficult to form.
An alternative approach has been the use of ion beams produced by ionising and accelerating argon clusters which are generated by supersonic expansion of argon gas through a nozzle. In this way, clusters of a few hundred argon atoms to a few tens of thousands of argon atoms can readily be formed. This has been widely used in SIMS and has been proposed for XPS in JP 08-122283 A, JP 2008-116363 A and WO 2009/131022. However, this technique has not been exploited commercially for application in XPS, which is believed to be due to the engineering difficulties of producing a focused beam with argon clusters of the correct size to produce a useful etch rate without destroying the sample chemistry.
Inert gas cluster sources in general have been widely used since the 1970's (see Hagena and Obert, Cluster Formation in Expanding Supersonic Jets: Effect of Pressure, Temperature, Nozzle Size and Test Gas, The Journal of Chemical Physics Vol 56 No 5 March 1972 p 1793). Commercially they are employed for the polishing of wafers. However, the focus of such commercial developments has been on the generation of high current ion beams (e.g. several microamps) using relatively large expansion nozzle sources typically bigger than 150 microns. This necessitates the use of large high speed vacuum pumps. Large inert gas clusters of several thousand atoms are typically preferred in these wafer polishing applications. Such gas cluster sources are not generally suitable for use with smaller vacuum systems as used with surface analysis systems such as XPS systems.
In view of the above, the present invention has been made.